This invention relates generally to the disposal of solid wastes and more particularly to the disposal of solid wastes in a rotary kiln.
As is known, many solid wastes may be disposed of by burning in a rotary cement kiln. Of particular importance is that the use of a cement kiln generally satisfies federal and state disposal regulations for solid hazardous waste and other regulated solid waste. The high temperatures developed in the kiln and the long exposure of the waste in the kiln to these elevated temperatures results in destruction of the organic components of the waste. The inorganic components of the waste combine with the active compounds of the cement, rendering them nonhazardous and recovering their material value in making a useful product. Further, the burning of combustible solid wastes can supply a portion of the energy requirement of the kiln for production of cement. Within certain limits, the introduction of the waste has no adverse affect on the quality of the cement produced by the kiln.
Briefly described, a typical rotary cement kiln includes a long (i.e., 200-600 ft.) cylindrical vessel which is inclined slightly from the horizontal. The vessel is approximately 9-20 ft. in diameter and is rotated slowly (about 1-3 revolutions per minute) about its axis. A mixture of finely ground cement raw materials is fed into the vessel at its elevated end. The materials travel slowly along the length of the vessel from the upper end to the lower end. A stream of heated gas is forced through the vessel from the lower end to the upper end, opposite the direction of flow of the mixture The gas temperature in the vessel is hottest at the lower end of the vessel and coolest at the upper end of the vessel. The thermal energy of the gases in the vessel drives the chemical reactions which convert the cement raw materials to cement clinker. In some cement kilns, solid fuel may be mixed with the raw materials. This fuel may provide additional thermal energy for the chemical reactions by combusting at a point along the length of the vessel where the gas temperature is sufficiently high. The intermixing of the solid fuel with the cement raw materials is believed to enhance the efficiency of the transfer of heat from the combusting solid fuel to the raw materials. There are other types of cement kilns, including those which have separate vessels for preheating or precalcining the cement raw materials. Reference is made to U.S. Pat. No. 4,022,629 and the references cited therein for a more detailed discussion of rotary cement kilns.
An example of a method and apparatus for disposal of solid hazardous wastes is shown in U.S. Pat. No. 4,850,290 (Benoit et al.). In Benoit et al., solid hazardous wastes are packaged in sealable steel pails or drums for transportation to the kiln. The waste in the containers preferably has an energy value of greater than 5.000 BTU/lb for any appreciable recovery of energy value in the kiln. Benoit et al. specifically call for introduction of the containerized solid waste to the kiln at a point along the length of the kiln where the gas temperature is between 950.degree. and 1200.degree. C., and preferably between 950.degree. and 1100.degree. C. These temperatures generally correspond to the calcining zone of the kiln in which calcium is freed for later recombination with components of other raw materials. This zone corresponds to approximately the central one third of the length of the kiln, assuming there is no preheating or precalcining of the raw materials prior to their introduction into the vessel. At a temperature range of approximately 1300.degree. to 1600.degree. C., the calcium released in the calcining zone recombines with other components of the raw materials to form cement clinker. According to Benoit et al., the hazardous waste should not be charged to the clinkering zone of the kiln because its introduction produces reducing rather than oxidizing conditions which adversely affects cement quality.
Benoit et al. provide a port in the side wall of the rotating, cylindrical vessel of the kiln, and a drop tube extending through the port for guiding containers of hazardous solid waste into the vessel. Because the drop tube is attached to and extends into the vessel, a special temperature resistant alloy must be employed in the construction of the tube. The portion of the tube inside the vessel must be covered by refractory material. The containers are raised up to the level of the drop tube and loaded into the portion of the tube outside the vessel as it passes by. In a second embodiment, the drop tube has two portions, a stationary vertical portion outside the kiln, and a portion inside the kiln which rotates with the kiln. The containers are charged to the kiln when the tube portions align. A cam operated door covers the port until the drop tube reaches a vertical (or aligned) position, at which time the door opens and the container drops into the vessel. Thus, the rate of charging containers to the vessel is limited by the rotational speed of the vessel.